function force = repulsive_force(obs, rob, k_rad, k_rep)
force = zeros(3, 1);

rho = norm(obs(1:3) - rob);
rho_0 = obs(4) * k_rad;
if rho < rho_0
    k = k_rep * (1 / rho - 1 / rho_0) * (1 / rho^2);
    force = force + k * (rob - obs(1:3)) / rho;
end